The present invention relates to a filter unit for filtering gas, comprising at least one filter element having an outlet opening facing a clean-gas chamber, a filter cleaning nozzle, and a source of pressurized primary cleaning gas.
A filter unit of this type is known from DE patent 197 01 983 describing a nozzle of the slit ring jet type where the nozzle opening is an annular slot, which faces radially inwards towards the center of a flow pipe which passes centrally through a pressurized gas chamber and ends above the filter outlet opening. The control valve is pneumatically activated and built into the nozzle at the annular slot. The primary cleaning gas flows from the gas chamber through the annular slot and down along the inside of the flow pipe which acts as guide surface. The secondary gas is drawn into the flow pipe through its upper opening. The cleaning gas passes through a venturi mounted in the upper end area of the filter member.
A somewhat similar nozzle is described in U.S. Pat. No. 4,356,010 and in GB patent 1 455 281, but in the latter document the ring slit jet nozzle is mounted like a venturi in the top end of the filter element. The secondary gas is drawn centrally down through the upper end opening of the nozzle. GB patent 812,244 describes a simple nozzle shooting primary gas down through a venturi so that secondary gas is drawn down through the upper end opening of the venturi. GB patent 1,021,560 describes a nozzle pipe which is mounted in the bottom wall of a pressurized gas chamber and is closed at its upper end by a diaphragm valve. When the diaphragm valve is opened primary cleaning gas flows through the nozzle pipe and centrally down through a venturi at the upper end of the filter element. U.S. Pat. No. 3,729,903 describes a ring jet filter with an annular membrane valve mounted in the upper end of the filter element. A central pipe connects the filter element with the clean-air chamber. When the membrane valve is open secondary air is also drawn down through the upper end opening of the central pipe.
A feature common to all the prior art filter units mentioned above is that the secondary air is drawn in through the upper end opening of a pipe located centrally of the filter element.